Offshore hydrocarbon drilling and production systems require a fluid connection between the subsea production system and the floating production storage and offloading vessel (FPSO). Upon arrival at the offshore wellsite, the FPSO can be secured to the seabed or dynamically positioned using an onboard propulsion system. A mooring system is used to couple the FPSO to the seabed. A spread mooring of the FPSO can ensure position control and a fixed heading of the FPSO at the sea surface. Various mechanisms and apparatus are used to connect the subsea production or manifold system to the FPSO for transferring hydrocarbons. A spread mooring connection will physically connect to the FPSO. A riser connection or hook-up will fluidicly connect the subsea production system to the FPSO. For example, the fluid connection between the subsea system and the FPSO can be used for hydrocarbon production, water injection, gas injection, chemical injection, control lines, and the like.
Typically, risers and spread mooring are located at the outside of the hull of the FPSO, such as at the port and/or starboard side, and are installed after the FPSO has arrived at the operation wellsite. The spread mooring system is typically installed in four mooring clusters from four locations on the vessel. The mooring lines can be made from chains and neutrally buoyant polyester rope. Pile or suction anchors can be used to fix the mooring and is typically preinstalled before FPSO arrival. The connection of the mooring lines is done after FPSO arrival at the wellsite with the help of construction vessels and positioning tug boats. After the mooring lines are handed over to the FPSO they are pre-tensioned using an onboard mooring tension system.
A riser balcony at a side of the FPSO is configured to receive and connect to the risers. The risers may be free hanging. The riser balcony typically includes a lower riser balcony, an upper riser balcony, and a pull-in balcony. The lower riser balcony is used to fix the risers laterally to the vessel through a riser bend restrictor. The upper riser balcony is used to fix the risers vertically. The pull-in balcony often includes a skidding rail or similar structure to allow the pull-in device and/or sheave to travel longitudinally to pull in/out each riser. The risers are pulled in individually with a handover from a flex lay vessel. The commissioning of the wells can start after the risers are pulled in and connected. The pull-in of risers is performed after all mooring lines are installed.
Deep waters with large quantities of risers routed directly to the FPSO result in high loads on the side of the FPSO hull. Normally risers are routed to only one side of the hull to allow safe approach for supply vessels at the opposite side. In order to maintain stability and buoyancy, extra ballast tanks and/or buoyancy may be required. However, extra ballast or void tanks will reduce the available cargo capacity or increase the required steel material needed. Alternative ways to reduce riser loads include using separate buoyancy solutions to support the riser weights, or reducing the quantity of risers through subsea manifolding. However, both of these options are costly and reduce operational flexibility. In addition to the reduction of cargo capacity in the FPSO, having the risers suspended from the side of the vessel also has a negative impact on the riser fatigue life. The support structures from which the risers hang are away from the vessel center, thereby creating a lever arm on the vessel and amplifying the vessel motions in the risers. The FPSO is subjected to rolling motion from the sea water, and wave slamming motion at the FPSO hull side where the risers are suspended. These two factors, vessel motion and wave slamming, introduce high load on the risers thereby decreasing riser fatigue life.